The preparation of overdentures involves a series of positive and negative impressions and casts which are used so as to adapt the overdenture to the shape of the oral cavity and so as to orient the fastener elements in the jaw and in the overdenture correctly in relation to each other. During this casting-impression procedure, a temporary coping is placed upon the ball abutments in the jaw and moulded into an impression in an impression spoon. The copings have a bore which is complementary to the ball parts of the ball attachment devices. The copings, which now are located in the impression, are then utilized to hold ball abutment replicas which are moulded into a positive model of the jaw cast into the impression. The snap fasteners or ball housings are then attached to the temporary ball abutment replicas and moulded into or otherwise attached to the overdenture.
The copings normally have an interior bore which is circular in cross-section. This means that the fit between the coping and the ball part has to be very exact. If the bore is too small, it will be difficult to press the coping onto the ball part, and if the bore is too wide, there will be no or insufficient friction to hold the ball abutment and the abutment replica securely. Furthermore, special arrangements have to be made in order to allow the air in the bore to escape, such as channels to the outside etc.
It thus is desirable that there is a specified friction between the bore in the coping and the ball parts of the respective ball abutment This sets a high standard in regard of the dimensional precision of the bore. It furthermore is desirable that any air present in the interior of the coping has a possibility of escaping when the coping is pushed down onto the ball part in order to allow the coping to be fully seated on the ball part.